The mentioned checking devices are, for example, automatic sorting, checking and/or counting apparatuses for sheet material such as in particular bank notes, checks, identification documents, share certificates/documents of value or other documents. Moreover, checking devices are concerned which can be generally used to check objects as to the presence or absence of certain properties. The further description relates by way of example to the first mentioned checking devices for sheet material.
So as to check the sheet material within such devices for example as to type, authenticity, validity or damage, there are usually used, among other things, optical sensor devices. In the sensor devices the sheet material is illuminated with special light sources, and the light reflected by the sheet material or transmitted through the sheet material is measured and evaluated by corresponding sensors. For spectroscopic examinations light with certain wavelengths and a radiant power as high as possible is required here.
A device for the lighting of sheet material is described in EP 0 762 174 A2. The lighting means here substantially consists of a plurality of mirror segments of cylindrical mirrors with elliptical base surface, which each have two focus lines. The mirror segments are arranged such that both the first and the second focus lines of all mirror segments are coincident in a first or second focus line, respectively. In the first common focus line there is arranged a light source consisting of a series of light emitting diodes. The light emitted by the light emitting diodes is focused by reflection at the mirror segments onto the sheet material located in the second common focus line, and so a line-shaped lighting for the sheet material with a relatively high illuminance is produced. This, however, is a relatively elaborate and sensitive set-up. Due to the use of light emitting diodes as a light source, the radiant power is limited.
To increase the detection sensitivity in the sensor device, however, higher radiant power is necessary. This is possible, for example, when more intense light sources, in particular laser light sources, in the visible (400 to 700 nm) and infrared (700 to 1400 nm) or ultraviolet (300 to 400 nm) range are used. Here, the use of laser diodes of the laser classes 3b and 4, of Nd:YAG lasers or other solid state lasers with continuous or pulsed radiation is particularly suitable. With the use of lasers, however, due to the high radiant power and the good focusability of the radiated light, there arises a relatively great risk for the eyes of operators or of service personnel of the apparatuses. Therefore, for conventional laser lighting there are applicable restrictive power limitations so as to ensure eye protection, which either allow an only very low laser power—which is very unhelpful for the intended use—or require extensive safety measurements.
A possibility to ensure the protection of employees is to provide the doors of a closed system with interrupt switches which lead to a switch-off of the laser when the doors are opened. In particular with the sheet material processing devices, however, it is often necessary to carry out service work with the doors open. This is the case for example when the transport path of the sheet material is set or places within the transport path have to be searched for where sheet material is damaged, or the sensor devices or parts thereof have to be cleaned because they were soiled by the processing of the sheet material. But this service work may possibly turn out to be very difficult, because due to the automatic switch-off of the laser light sources in the sensor device, the conditions present during normal operation are changed. For example, in such a case it is very difficult for the service personnel to recognize that, for example, the sensor device or parts thereof are soiled and have to be cleaned.